HADOOP-16490 add an extra assert to ITestS3GuardTtl

Gabor reported an NPE here on a test run of this PR only; added asserts that the lists from the MS are never null

Change-Id: I9154eadabb486f601f86e51121d2f05d912c8e46

hadoop-tools/hadoop-aws/src/test/java/org/apache/hadoop/fs/s3a/ITestS3GuardTtl.java

@@ -18,6 +18,7 @@
 
 package org.apache.hadoop.fs.s3a;
 
+import java.io.IOException;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
@@ -331,7 +332,7 @@ public void testListingFilteredExpiredItems() throws Exception {
 
       // listing will contain the tombstone with oldtime
       when(mockTimeProvider.getNow()).thenReturn(oldTime);
-      final DirListingMetadata fullDLM = ms.listChildren(baseDirPath);
+      final DirListingMetadata fullDLM = getDirListingMetadata(ms, baseDirPath);
       List<Path> containedPaths = fullDLM.getListing().stream()
           .map(pm -> pm.getFileStatus().getPath())
           .collect(Collectors.toList());
@@ -342,7 +343,8 @@ public void testListingFilteredExpiredItems() throws Exception {
 
       // listing will be filtered, and won't contain the tombstone with oldtime
       when(mockTimeProvider.getNow()).thenReturn(newTime);
-      final DirListingMetadata filteredDLM = ms.listChildren(baseDirPath);
+      final DirListingMetadata filteredDLM = getDirListingMetadata(ms,
+          baseDirPath);
       containedPaths = filteredDLM.getListing().stream()
           .map(pm -> pm.getFileStatus().getPath())
           .collect(Collectors.toList());
@@ -356,4 +358,14 @@ public void testListingFilteredExpiredItems() throws Exception {
     }
   }
 
+  protected DirListingMetadata getDirListingMetadata(final MetadataStore ms,
+      final Path baseDirPath) throws IOException {
+    final DirListingMetadata fullDLM = ms.listChildren(baseDirPath);
+    Assertions.assertThat(fullDLM)
+        .describedAs("Metastrore directory listing of %s",
+            baseDirPath)
+        .isNotNull();
+    return fullDLM;
+  }
+
 }